Typically, 75W-140 is common for heavier-duty applications or colder climates, while 75W-90 might be specified for lighter-duty applications or warmer climates. Some F-150s even used a 80W-90.
To find the correct weight:
1. Check your owner's manual: This is the most reliable source of information. It will specify the recommended fluid weight based on your truck's specific configuration.
2. Look for a sticker on the differential: Sometimes there's a sticker on the differential cover indicating the recommended fluid type and weight.
3. Check the differential tag: This tag, usually located on the differential itself, might list the axle ratio. This, combined with your owner's manual or a Ford parts lookup based on your VIN, can help you find the correct fluid weight.
Using the incorrect weight can damage your differential. Always consult your owner's manual or a qualified mechanic for the correct specification.